CN101136892A - Whole phase FFT based OFDM modulation-demodulation method - Google Patents

Abstract

The modulation demodulation method includes following steps: after IFFT, carrying out precoding in full phases to generate needed signal including full phases of all sampled points plus cycle prefix; the receiving end carries out process in full phases for the received signal; using circuit of full phases algorithm to process sampled data; using technique of frequency spectrum correction (FSC) to carry out FSC so as to obtain correct sub carrier amplitude, frequency; using planisphere carries out decoding to obtain digital binary code finally. Advantages are: the technique can obtain carrier phase without error; when frequency deviation occurs, the method causes power leakage much less than leakage from FFT demodulation; the method can omit training sequence added in order to keep synchronization; the method can reduce influence on system by noise, about 1/3 lower than influence caused by FFT.

Description

OFDM modulation-demo-demodulation method based on whole phase FFT

Technical field

This patent belongs to the modulation-demodulation technique field, further relates to a kind of OFDM modulation-demo-demodulation method based on whole phase FFT.

Background technology

The basic principle of OFDM technology

OFDM is the high speed transmission technology that is applicable under the wireless environment, except WLAN standard (IEEE802.11a, HiperLAN/2), also is applied in broadband wireless access (BWA).

Fig. 1 is the theory diagram that ofdm system is used in mobile communication.Fig. 2 is its realization figure.

At transmitting terminal, at first initial data is encoded, interweaved, carry out serial to parallel conversion then, one road signal is divided into parallel N road, data-modulated parallel transmission to a plurality of mutually orthogonal subcarriers, the N that obtains after an IFFT conversion sampling point is called an OFDM symbol by N point IFFT conversion, then last L sampling point of symbol copied to the foremost, as CP, be used to resist ISI, send by transmitter again.Receiving terminal is carried out the process opposite with transmitting terminal, to radio frequency (RF, RadioFrequency) the laggard line sampling of signal down-conversion, the sampling point that obtains dispersing, carry out the original position that timing estimation finds the OFDM symbol then, remove the CP part, N sampling point of CP back done N point FFT conversion, adjudicate demodulation then, if employing coherent demodulation, need so also to estimate that channel parameter comes aid demodulation, data after demodulating is carried out deinterleaving, decoding, obtains original data.

As shown in Figure 3, be the example that comprises N subcarrier in the OFDM symbol.Wherein all subcarriers all have identical amplitude and phase place, but in actual applications, according to the modulation system of data symbol, it is impossible that each subcarrier all has identical amplitude and phase place.As can be seen from Figure 3, each subcarrier comprises an integral multiple cycle in an OFDM symbol period, and differs one-period between each adjacent subcarrier, and this characteristic can be used for explaining the orthogonality between the subcarrier, that is:

$\frac{1}{T}{\&Integral;}_0^T\mathrm{exe}\left(j{w}_{n}t\right)\exp \left(j{w}_{m}t\right)\mathrm{dt}=\left\{\begin{array}{c}1,m=\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="A20071006139300131.png,A20071006139300171.png"\; state="\{\{state\}\}">n</figure-callout>}\\ 0,m\&NotEqual;n\end{array}---\left(1\right)\right.$

This orthogonality can also be explained from the angle of frequency domain.According to formula (1), each OFDM symbol comprises the subcarrier of a plurality of non-zeros in its period T, thus its frequency spectrum can to see as be to be frequency spectrum and one group of convolution that is positioned at the δ function on each sub-carrier frequencies of the rectangular pulse of T in the cycle.The spectral magnitude of rectangular pulse is sinc (fT) function, and this zero of a function appears on the position that frequency is the 1/T integral multiple.As shown in Figure 4.The sinc spectrum of function of the symbol that moulding obtains through square waveform in each subchannel.At the maximum place of each sub-carrier frequencies, the spectrum value of every other subchannel is zero just.Owing to the OFDM symbol is being carried out in the process of demodulation, need to calculate each subcarrier and only get the pairing letter in peaked position value early, therefore can from a plurality of overlapped subchannel symbol frequency spectrums, extract each subchannel symbol, and can not be subjected to the interference of other subchannels.As seen from Figure 4, the Nyquist difficulty that in fact frequency spectrum of OFDM symbol can satisfy no intersymbol interference then, but traditional Nyquist criterion is to send noiseless shadow between the symbol before and after guaranteeing on the time domain, disturb but refer to herein in frequency domain, not to be present on each subchannel, the method of disturbing (ICI) between this elimination subchannel is that take a sample and realize in the maximum place of each subcarrier in frequency domain by use rectangular pulse moulding in time domain.

The realization of DFT

Fourier transform links together time-domain and frequency-domain, and the form of Fourier transform has several, selects the concrete environment decision of the Fourier transform of any form by work.Most of signal processing are used DFT.DFT is a kind of variation of conventional conversion, and wherein, signal is all sampled on time domain and frequency domain.By the definition of DFT, waveform repeats continuously on the time, therefore causes the frequency spectrum on the frequency domain to repeat continuously.Fast fourier transform (FFT, Fast Fourier Transform Algorithm) only is a kind of quick mathematical method of DFT computing application, because its high efficiency makes the OFDM development rapidly.

$s_k=s(\mathrm{KT}/N)=\underset{i-0}{\overset{N-1}{\mathrm{\&Sigma;}}}{d}_i\exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20071006139300151.png,A20071006139300171.png"\; state="\{\{state\}\}">j</figure-callout>}\frac{2\mathrm{\&pi;ik}}{N}\right)$

(0≤k≤N-1) (2)

Can see s _kEquivalence is to d _iCarry out the IDFT computing.Equally at receiving terminal, in order to recover raw data symbols d _i, can be to s _kCarry out inverse transformation, i.e. DFT:

$d_i=\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{d}_i\exp (-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20071006139300151.png,A20071006139300171.png"\; state="\{\{state\}\}">j</figure-callout>}\frac{2\mathrm{\&pi;ik}}{N})$

(0≤i≤N-1) (3)

Can see that according to above analysis the modulation and demodulation of ofdm system can be replaced by IDFT and DFT respectively.The IDFT computing of ordering by N is frequency domain data symbol d _iBe transformed to time domain data symbol s _k, after the radio-frequency carrier modulation, send.The data symbol s of each IDFT output wherein _kAll be that all sub-carrier signals generate through stack.In the practice of ofdm system, can adopt more convenient IFFT/FFT.N point IDFT computing need be implemented N ²Inferior complex multiplication, and IFFT can significantly reduce complexity.

Protection interval and Cyclic Prefix

(1) protection at interval

The important feature of OFDM is can effectively avoid because the intersymbol interference that multipath causes.In the parallel subchannel of N, make the input traffic serial to parallel conversion N that the cycle data of each modulated sub-carriers can expand the artificial raw data symbols cycle doubly, so the numeric ratio of time delay expansion and symbol period reduce N doubly too.Adding protection (GI, Gard Interval) at interval in order to eliminate intersymbol interference between the OFDM symbol, can not insert any signal in the GI, promptly is the transmission period of one section blank.Yet in this case because the influence of multipath transmisstion can produce ICI, can in GI, insert under a lot of situations Cyclic Prefix (CP, CyclicPrefix).Promptly last several sampling points of OFDM symbol are copied to the foremost of symbol.Like this, when the maximum delay time of channel during less than the length of CP, the interference between the OFDM symbol only occurs in the CP interval, and the number of cycles of the subcarrier that is comprised in the OFDM time-delay copy also is integer, by after removing the CP part, can make the OFDM symbol not be subjected to the influence of ISI at receiving terminal.CP is before sending, and it is added into the front end of OFDM symbol.Schematic diagram such as Fig. 5, Fig. 6:

But.Ofdm system adds after the GI, the loss that can bring power and information rate, and wherein power loss is

${\mathrm{\&eta;}}_{\mathrm{cp}}=10l{\mathrm{og}}_{10}(\frac{L}{N}+1)---\left(4\right)$

When GI accounted for 20%, power loss can not surpass 1dB, but the information loss rate of bringing is up to 20%.

(2) Cyclic Prefix

Because the ICI that causes of multipath transmisstion, a kind of effective method is to be that the OFDM symbol of T carries out the cycle expansion with original width, fills protection at interval with spread signal in order to eliminate. as shown in Figure 7.(duration will be used T in will protecting at interval _gThe expression) signal be called Cyclic Prefix (Cyclic Prefix, CP).As can be seen from Figure, signal in the Cyclic Prefix and OFDM symbol tail width are T _gPart identical.The OFDM symbol at first will add Cyclic Prefix before sending into channel in real system, sent into channel then and transmitted.At receiving terminal, at first the width that receiving symbol is begun is T _gPart abandon, be the conversion when carrying out Fourier of the part of T then with remaining width, carry out demodulation then.Add Cyclic Prefix and can guarantee that in the FFT wave period number that the time delay copy of .OFDM symbol is comprised in the cycle also be integer in the OFDM symbol, like this, time delay is less than the protection interval T _gDelay time signal just can in demodulating process, not produce and not compare ICI.Fig. 8 has provided the example of the influence that a multipath transmission causes the OFDM symbol.Channel among the figure is two footpath fading channels, and solid line represents to arrive through the 1st paths the signal of receiving terminal, and dotted line is represented the delay time signal through the solid-line signals of the 2nd paths arrival.In fact, the signal that the OFDM receiver is seen is all signal sums, for the influence of multipath clearly is described, has provided each sub-carrier signal among the figure.The subcarrier of OFDM has adopted the BPSK modulation as we can see from the figure, and promptly at the boundary of symbol, the saltus step of 180 degree might take place carrier phase.For the dotted line signal, this phase hit can only occur in after the solid-line signals phase hit, so and since the hypothesis multidiameter delay less than protection at interval. can guarantee in the operation time of FK length, the saltus step of signal phase not to take place.Therefore, what the OFDM receiver was seen only is to have some phase deviation, a plurality of simple continuous sine-shaped superposed signals, and this stack can not destroy the orthogonality between the subcarrier yet.If yet multidiameter delay has surpassed protection at interval; then owing to may occur the saltus step of signal phase in FFT length operation time; thereby therefore just no longer include only simple sine wave-shaped signal continuously in the superposed signal of the 1st path signal and the 2nd path signal. cause the orthogonality between the subcarrier to be destroyed, cause ICI.

(3) ofdm system behind the adding Cyclic Prefix

Add behind the Cyclic Prefix as shown in Figure 9 based on the ofdm system block diagram of IDFT (IFFT).

Synchronism deviation is to the ofdm system Effect on Performance

The frequency response curve of wireless channel is non-flat forms mostly, and the main thought of OFDM technology is exactly the subchannel that given channel is divided into many quadratures in frequency domain, on each subchannel, use a subcarrier to modulate, and each subcarrier parallel transmission, like this, although total channel is a non-flat forms, has frequency selectivity, but each subchannel is a relatively flat, what carry out on each subchannel is narrow band transmission, signal bandwidth is less than the respective bandwidth of channel, so the just interference between the erasure signal waveform greatly.Because the carrier wave of each subchannel is mutually orthogonal in ofdm system, their frequency spectrum is overlapped, so not only reduce the interference between subcarrier but also improved the availability of frequency spectrum again, and as shown in Figure 4, be the spectrum diagram of ofdm system.

(1) influence that brings of carrier synchronization deviation

In the ofdm system, the parallel transmission of N symbol can make the perdurabgility of symbol longer, and therefore, it is insensitive to the deviation of time.For radio communication, there is time variation in wireless channel, exists the frequency shift (FS) meeting that the orthogonality between the ofdm system subcarrier is destroyed in transmission, and phase noise also has very big infringement to system.The skew of frequency can make to produce between the subchannel and disturb.The output signal of ofdm system is the stack of the subchannel of a plurality of mutual coverings, and the orthogonality of antithetical phrase interchannel has strict demand.A kind of imbody of wireless channel time variation is exactly a Doppler frequency shift, and Doppler frequency shift is directly proportional with the translational speed of carrier frequency and travelling carriage.Doppler broadening can cause frequency generation disperse, and signal is distorted, and from frequency domain, distorted signals can be aggravated with the increase of the Doppler broadening of transmitting channel.Therefore for the ofdm system that requires the subcarrier strict synchronism, the frequency shift (FS) of carrier wave brings more, and to influence meeting more serious, if do not take measures this interchannel interference (ICI) is overcome, systematic function is difficult to improve.As shown in figure 10, be that frequency deviation spectrum diagram afterwards takes place.Because the existence of frequency deviation, the sample value of sampling can be subjected to closing on the interference of carrier wave, that is to say, sampled point is not at the zero point of facing ripple, and (solid line is the actual samples point among the figure) and this interference are very large.So carrier synchronization and frequency offset correction are very important.

Carrier Synchronization Algorithm

In fact frequency departure decomposes two parts: integer part and fractional part.So-called integer part is that part of deviation that equals the integral multiple of subcarrier spacing; Fractional part is meant that a part of frequency departure less than subcarrier spacing.Integer part only makes information signal translation on subchannel, does not destroy the orthogonality between each subcarrier, but it causes the complete mistake of whole demodulation result, and error rate of system is almost 0.5; Fractional part then can cause subchannel to disturb, and destroys the orthogonality between each subcarrier, causes the error rate to increase.

Ofdm system estimates that the method for frequency deviation has three classes: data householder method (training symbol method); Non-data householder method (blind algorithm) is utilized the characteristic of the received signal in the frequency domain; The Cyclic Prefix estimation technique based on OFDM.The realization that system realizes synchronized algorithm specifically as shown in figure 11, thereby as can be seen: carrier synchronization is crucial.

Wherein data auxiliary (data-aided) are estimated, the advantage of this class algorithm be catch fast, precision is high, but can lose certain system effectiveness, be fit to block data communication.Non-data are assisted (non-dataaided), it is blind estimation, it utilizes the structure of ofdm signal, for example make the front end of OFDM and rear end that certain correlation be arranged, utilize empty subcarrier to do estimation, and utilize data to do estimation through the methods such as cyclostationarity after the molding filtration owing to adding Cyclic Prefix.The great advantage of blind estimation is: avoid owing to inserting the wasting of resources that frequency pilot sign brings.Shortcoming is to need tens even a hundreds of OFDM piece in order to obtain high accuracy, and capture time is long, and this class algorithm is suitable for descending cellular system, broadcast system, because the receiver signal of experience down channel always.The Cyclic Prefix estimation technique based on OFDM: the architectural feature of utilizing the OFDM symbol---Cyclic Prefix is estimated.No matter which kind of method of use all can not be eliminated the carrier wave asynchrony phenomenon fully, also just can not eliminate ICI fully.

Full phase place principle

The proposition and the development thereof of full phase analysis method, its essential idea produces truncated error in order to solve signal data and to block and proposes.The blocking artifact, one-dimensional signal that picture signal is blocked generation is because of blocking the spectral leakage that produces and interval signal reconstructed error etc. when doing spectrum analysis.As Figure 12, full phase digital signal data preliminary treatment can be simply by following some describe: suppose an one-dimensional signal is blocked discretization afterwards, its data length is 2N-1, with this segment data data segment that to be divided into N data length be N, and that is to say the N every segment data cyclic shift alignment of naming a person for a particular job with the central sample point, the data that successively the superimposed back normalization in the position of correspondence to be obtained one section sample length then be N.This shows, full phase method for position mainly be by considered data segment central sample point might make up the problem that the truncated error that reduces signal is introduced.

The system of supposing research is a linear time invariant system, and then it satisfies homogeneous property, additivity and timeinvariance.Suppose that each N segment data is that arbitrariness is blocked, and also promptly adds rectangular window and blocks.As Figure 12, the situation of blocking for each N segment data of considering its central sample then needs N segment data vector altogether.This N segment data vector can be obtained the data vector that a segment length is N after with the stack of x (N) alignment cyclic shift.It is the pretreated basic thought of full phase data.Figure 13 is that the whole phase FFT system of N=4 realizes circuit diagram.

Summary of the invention

The present invention aims to provide a kind of reasonable method more and carries out the demodulation of OFDM, makes carrier synchronization be reduced to minimum to the influence of system with minimum cost.

A kind of OFDM modulation-demo-demodulation method based on whole phase FFT comprises the steps:

The first step is carried out full phase place precoding and is generated needed signal after IFFT, this signal must meet the requirement of full phase place to sample point, promptly will comprise whole phase places of all sample points, and then add Cyclic Prefix.Figure 14 has shown its operating process.

In second step, next need carry out full Phase Processing to the received signal at receiving terminal.Carry out full phase sample at first to the received signal, 2N-1 the sample point that needs are handled extracts from sequence then.

The 3rd step, sampled data through full phase algorithm processing of circuit, promptly according to Figure 12 2N-1 point carry out cyclic shift, stack obtains N sample point, then this N point is done the accurate phase place that the FFT processing can obtain N the subcarrier that we want.

In the 4th step, utilize the frequency spectrum alignment technique to carry out the frequency spectrum correction and obtain sub carrier amplitude accurately, frequency.

The 5th step, utilize planisphere to decipher, finally obtain the digital binary sign indicating number.So far finished the modulation and demodulation process of full phase place OFDM.

The present invention has following advantage with respect to existing FFT:

(1) this technology can obtain free from error carrier phase;

(2) when frequency deviation took place, the power leakage that it causes was well below the leakage of FFT demodulation;

(3) whole phase FFT can carry out frequency offset correction;

(4) whole phase FFT can save the training sequence that adds synchronously because of keeping;

(5) can reduce the influence that grass is given system simultaneously during full phase demodulating, than FFT low 1/3.Use the cost of full phase place to remedy at transmitting terminal.

Description of drawings

Fig. 1 ofdm system theory diagram;

Fig. 2 ofdm system basic model block diagram;

The situation that comprises six subcarriers in Fig. 3 OFDM symbol;

Fig. 4 ofdm system sub-channels symbol frequency spectrum;

The circulating prefix structure figure of Fig. 5 OFDM;

Under Fig. 6 multipath situation, the interference that idle protection causes at subcarrier at interval;

Fig. 7 has the Cyclic Prefix in OFDM System signal;

The transmission of Fig. 8 multipath is to the influence of ofdm signal;

Fig. 9 adds the ofdm system block diagram that utilizes IDFT/DFT to implement after the protection at interval;

Sampled point schematic diagram after Figure 10 generation frequency deviation;

Figure 11 ofdm system synchronized algorithm block diagram;

The data processing schematic diagram of the full phase place of Figure 12;

Figure 13 N=4 whole phase FFT circuit diagram;

The processing of the full phase place OFDM originating terminal signal of Figure 14;

Figure 15 adds the ofdm system optimum implementation figure after the full phase algorithm;

Figure 16 tradition OFDM demodulation planisphere;

The full phase place ofdm system of Figure 17 demodulation planisphere;

Embodiment

The process of front and traditional OFDM are just the same, and (below be the specific implementation process of full phase algorithm) carries out the IFFT conversion on N rank then, obtain N subcarrier; Carry out full phase place precoding, promptly symbol duplicates; Loaded into transmit is gone out after inserting Cyclic Prefix; At first remove Cyclic Prefix to the received signal; Each OFDM symbol is carried out two prototypes that sampling processing obtains being equivalent to a symbol of former OFDM; The sample value that obtains is used full phase place operation transform, obtain final N to be processed sample point; N sample point adopted the FFT conversion, obtain phase place, amplitude and the frequency of N subcarrier, carry out frequency offset correction simultaneously; Hint obliquely at by constellation and to obtain corresponding code element.

In frequency deviation is 1.003, and carrier frequency is 50KHz, and modulation system is 4PSK, the demodulation planisphere contrast when sample frequency is 10MHz, and in order to prove the advantage of full phase place to frequency offset correction, native system does not add interchannel noise.Figure 16 is the planisphere of traditional ofdm system demodulation, and Figure 17 is the ofdm system demodulation planisphere that adopts whole phase FFT.Obviously find out: adopt the systematic function after the full phase place to be better than traditional ofdm system.

Claims (1)

1. the OFDM modulation-demo-demodulation method based on whole phase FFT comprises the steps:

The first step is carried out full phase place precoding and is generated needed signal after IFFT, this signal must meet the requirement of full phase place to sample point, promptly will comprise whole phase places of all sample points, and then add Cyclic Prefix;

In second step, next need carry out full Phase Processing to the received signal at receiving terminal; Carry out full phase sample at first to the received signal, 2N-1 the sample point that needs are handled extracts from sequence then;

The 3rd step, sampled data through full phase algorithm processing of circuit, promptly according to Figure 12 2N-1 point carry out cyclic shift, stack obtains N sample point, then this N point is done the phase spectrum accurately that the FFT processing can obtain N the subcarrier that we want;

In the 4th step, utilize the frequency spectrum alignment technique to carry out the frequency spectrum correction and obtain sub carrier amplitude accurately, frequency;

The 5th step, utilize planisphere to decipher, finally obtain the digital binary sign indicating number; So far finished the modulation and demodulation process of full phase place OFDM.

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